Recent communication systems such as mobile phones need to support various communication standards and furthermore even with the same communication standard the frequency band differs depending on the country and region, so that the respective standards consist of a plurality of frequency bands. In such a communication system, among others a SAW (Surface Acoustic Wave) device, in which interdigital electrodes for exciting a surface acoustic wave are formed on a piezoelectric substrate in order to adjust and select a frequency, is in use.
For this surface acoustic wave device, a piezoelectric material such as lithium tantalate (LiTaO3; LT) and lithium niobate (LiNbO3; LN) is used to make the base substrate, because piezoelectric materials meet the requirements of small size, small insertion loss, and ability to stop passage of unnecessary waves. In particular, communication standards of cellular phones of the third generation and beyond often have a narrow differential in frequency band between transmission and reception, and a wide bandwidth, but on the other hand, the piezoelectric substrate material used conventionally for surface acoustic wave devices changes its characteristics depending on the temperature, so that the frequency selection range may be shifted, resulting in problems in the functions of the filter and duplexer. Therefore, a material for a surface acoustic wave device having small tendency to undergo fluctuation in characteristics with respect to temperature change and having a wide band is on demand.
Regarding the material for the surface acoustic wave device, for example, IP Document 1 teaches that a stoichiometric composition LT composed of copper used as an electrode material and mainly obtained by a gas phase method is preferable because the breakdown mode which is destroyed at the moment when high power is input to the IDT electrode is difficult to occur. IP Document 2 has a detailed description on the stoichiometry composition LT obtained by the gas phase method; and IP Document 3 describes a method of forming a waveguide for annealing a waveguide formed in a ferroelectric crystal of lithium tantalate or lithium niobate; and IP Document 4 describes a piezoelectric substrate for a surface acoustic wave device obtained by subjecting a lithium tantalate or lithium niobate single crystal substrate to Li diffusion treatment. IP Document 5 and Non-IP Document 1 also report that when LT in which the LT composition is uniformly transformed to Li-rich in the thickness direction by the gas phase equilibrium method is used to make the surface acoustic wave element, its frequency temperature characteristic is improved, which is preferable.
However, the inventors of the present invention have examined the methods described in these publications, and as a result, it has been found that these methods do not necessarily provide favorable results. In particular, according to the method described in IP Document 5, since the wafer is processed over a long period of time of 60 hours at a high temperature of about 1300° C. in the vapor phase, the manufacturing temperature has to be high, the consequent warpage of the wafer is large, and cracks occur at high rate, whereby the productivity becomes poor, and there is also a problem that the product becomes too expensive as a material for a surface acoustic wave device. Moreover, the degrees of variation in characteristics become large on account of the facts that the vapor pressure of Li2O is low and the modification degree of the sample to be modified varies depending on the distance from the Li source, and hence a considerable improvement is required for industrialization, and this problem has not been solved yet.